Plug Weld Question

I am replacing a panel which had been spot welded in place. I want to plug weld the piece by punching holes in the overlapping piece and starting the weld on the underlying piece to get a plug weld. The material is 16 gauge sheetmetal. I will be using a 110 Miller mig with .023 wire. Based on the above information, what size hole should I punch in the overlapping metal? Thanks for your expertise.

 

i'd say around 4-5mm. the trick to getting a good plug weld is to "zap" your wire on a scrap just before you start each weld. because the previous weld often leaves a little "dab" on the end of the wire, and you have to get the heat up quick in order to get some penetration before your hole is full, you need to start with as much heat as possible. so zap a piece of scrap and while the wire is still nice and hot, weld the plug.

lather, rinse and repeat...

 

Personally, I use a letter A (.234), but I also run my welder a bit hot & a bit faster wire feed, set up for dot welding. I have heard some use as large as a 5/16" hole, but with my welder setting, it would likely blow thru from the additional heat. So I guess the point I'm trying to get across is, it depends on your heat setting, and technique. If you run your machine on the hot side, you can get by with a smaller hole. If you have it set for the ga. metal you are using, sometimes the start of the weld is a little cold (no penetration) so you'll need the slightly larger size to insure a good weld. I would suggest a couple practice pieces, in varying size holes, and conduct some "separation" tests, see how well it did. But somewhere in the 1/4" range would be a good start.

 

I use a 1/4 diameter hole for 16 or 18 gauge and a 3/16 diameter for 20 or 22 gauge.

 

thats a nice looking panel, what die did you use in your roller to get that crease?

 

I use 3/16" and clip the dingleberry off the wire for each one. clipping the berry give more heat time before filling.

 

I like 5/16 at most, but I use a spot weld drill to detach the old panel, so I usually have a bunch of holes that size already. That size cutter seems more durable than the smaller ones. I also like to weld in a flower petal pattern around the outside before filling the center.

 

the collision repair industry recomends a 5/16" hole,any smaller and you cannot get enough penetration

 

....hence my reply of basing the hole size on your heat setting. I have my machine set on the hot side using a smaller hole than 5/16" and it has plenty of penetration. I use the same heat setting as when I butt weld, and that process shows full weld penetration.

 

Remember that a butt weld generally also has the pieces butted at around half material thickness, so your gap will make a difference on penetration also.

 

Sorry, but butt welding in my shop does not generally have the gap you describe. The one joint you see above with the gap is not my normal work, I had to keep the outside edge aligned as I trimmed too much. Typically I have them butted tight together, like the top joint. I don't like to use a gap with butt welding as it gives you more room for shrinkage and thus the panel moving. (FYI, when finishing the fender pictured above, there was more panel distortion in the gapped seam than the tight one) IMO a tight butt weld removes most of this issue, and the little bit it does move can be overcome with a slight bit of hammer and dolly work. I rather use a hotter setting to insure I have full weld penetration, and don't rely on filling in a gap.

The point I was trying to make in the earlier post is that the heat setting on your welder will overcome any weld penetration issue you may find with smaller diameter holes. IMHO a plug weld with proper penetration (heat setting) will hold better than a spot weld. So why should we have to use a plug weld that is larger in diameter than the factory spot welds?

 

Penetration is accomplished through the heat and speed settings on the welder a 1/4" hole will be sufficient. My general rule is 1/4" holes about 3-5" apart with a few 5/16" holes on the ends. This would be for quarters, box sides, and roof skins. I always grind the spot welds on the old panel so I don't have hole to fill in the inner structure. Also, always start in the center and work your way out.

 

1/4 - 5/16 usually, did one recently at 5/16 cause the factory holes were similar once cut out.

 

If you are butt welding with a mig welder you should leave a gap the thickness of the metal being used otherwise you will have a mess on your hands. Hammer welding will accept the smaller gap but that is a totally different process. You can't work a mig weld it is too brittle.

Factory welds are done with pressure with no pre drilled holes if that is what some are thinking. A good spot welder can go through 4 sheets of metal without a problem giving perfect penetration and hold.

1/4" should be fine. To set up your welder take scrap and count or set pulse to where you have penetration on back of panel and small concave spot on top side. If you are set up right you can hit that hole dead center hold it and stop to reveal an almost finished weld needing minimal clean up. Of course you also have to have heat and wire speed adjusted. The idea is not to have big popcorn weld that needs to have the shit grinded out of it.

 

Same here but I think they're being cautious and catering to the lowest common denominator. They also specify copper spray in all lap joints to inhibit corrosion and I've had an inspector make me open a seam to see it.